Cold rolled steel strip with improved drawing properties and method for producing same

ABSTRACT

Killed steel containing 0.04-0.06 wt. percent carbon, and titanium at least six times the combined carbon and nitrogen contents, is subjected to controlled rolling and annealing steps to improve the drawing properties of the resulting cold rolled strip. No degassing of the molten steel. No decarburizing during annealing.

Unite 10i States Patent 1191 Hughes Dec. 10, 1974 1 vCOLD ROLLED STEELSTRIP WITH 3,333,987 8/1967 Schrader 148/2 IMPRQVED DRAWING PROPERTIESAND 3,492,173 l/1970 Goodenow 148/12 3,560,270 2 1971 Reinhold et a1.148/12 METHOD FOR PRODUCING SAME 3,607,456 9/1971 Forand, Jr. 1. 148/12Inventor: lan 1F. Hughes, Munster, 1nd.

Inland Steel Company, Chicago, 111.

Feb. 15, 1973 Assignee:

Filed:

Appl. No.: 332,659

Related US. Application Data Continuation of Ser. No. 130,496, April 1,1971, abandoned US. Cl. 148/12 C, 148/36 Int. Cl C2ld 9/48, C22c 39/54Field of Search 148/12, 36

References Cited UNITED STATES PATENTS 5/1965 Ohtake et a1. 75/49Primary Examiner-W. Stallard Attorney, Agent, or Firm-Merriam, Marshall,Shapiro & Klose [57] ABSTRACT Killed steel containing 0.04-0.06 wt,percent carbon, and titanium at least six times the combined carbon andnitrogen contents, is subjected to controlled rolling and annealingsteps to improve the drawing properties of the resulting cold rolledstrip. No degassing of the molten steel. No decarburizing duringannealing.

16 Claims, N0 Drawings 1 COLD ROLLED STEEL STRIP WITH IMPROVED DRAWINGPROPERTIES AND METHOD FOR PRODUCING SAIVE This is a continuation, ofapplication Ser. No. 130,496, filed Apr. 1, 1971, and now abandoned.

BACKGROUND OF THE INVENTION The present invention relates generally to amethod for improving the drawing properties of steel strip, and moreparticularly to a method for improving the drawing properties of killed,cold rolled steel strip containing between 0.04 and 0.06 wt. percentcarbon.

Heretofore, it has been conventional to improve the deep drawingproperties of killed, cold rolled steel strip by adding titanium to thesteel. However, to obtain the desired deep drawing properties whentitanium was added, it was also necessary that the carbon content of thesteel be less than 0.02 wt. percent (see Shimizu, US. Pat. No.3,522,110). To obtain this low carbon content, it was necessary tosubject the steel to a vacuum degassing operation while the steel wasstill in a molten condition or that the steel be decarburized after itwas rolled into strip.

The deep drawing properties of steel strip are expressed as the 7 value.The higher the F value, the better the deep drawing properties of thesteel. The T value is the average plastic strain ratio of the material,i.e., it reflects the ratio of percent change in width to percent changein thickness when the strip is pulled to a greater length (see Goodman,U.S. Pat. No. 3,404,047). It is desirable to produce killed, cold rolledsteel strip having an Fvalue greater than 1.9.

SUMMARY OF THE INVENTION A method in accordance with the presentinvention produces killed, cold rolled steel strip containing titaniumand having an Tvalue greater than 1.9, but without a vacuum degassingoperation or a decarburizing operation and with a carbon content-between0.04 and 0.06 wt. percent. The method comprises controlling the rollingand annealing operations, as described more fully below.

DETAILED DESCRIPTION INCLUDING A PREFERRED EMBODIMENT Steel stripproduced in accordance with the present invention has the followingtypical composition, in wt. percent:

carbon 0.04 0.06 manganese 0.29 0.35 nitrogen 0.003 0.007 titanium 0.300.65 aluminum 0.026 0.040 iron and essentially the usual balanceimpurities Manganese may be present in the permissible range 0.02-0.50.Nitrogen is generally present as an unavoidable impurity and may bepresent in amounts less than 0.015. The titanium content should be atleast six times the combined carbon and nitrogen content. Aluminum ispresent as a result of killing the steel with aluminum.

Molten steel of the above noted composition is prepared without vacuumdegassing, killed in the conventional manner and cast into ingots orslabs. The ingots are broken down into slabs using conventional rolling2 procedures. The slabs are hot rolled into hot rolled strip. The hotrolling procedure is essentially conventional up to the finishingtemperature.

The hot roll finishing temperature is at least l,645F.

(l,670l,7l0F. preferred) The hot rolled steel strip is then coiled whileat a temperature exceeding l,100F. l ,250l ,290F. preferred).

After hot rolling, the strip is cold rolled with a minimum coldreduction of about percent (preferably -70 percent).

Following cold rolling, the steel strip may be either batch annealed oropen coil annealed. In batch annealing, a coil of cold rolled steelstrip is heated at a slow heating rate (e.g., 30"-70F. per hour) to anannealing temperature greater than about 1,285F. (l,322l ,350F.preferred) and soaked at this temperature for at least 20 hours (25-30hours preferred).

In open coil annealing, the steel strip is heated, withoutdecarburizing, in an inert atmosphere, at the same slow heating rate asdescribed above in connection with batch annealing, to a temperature inthe range l,500-l,640F. (l,590-l,620F. preferred) for more than 5 hours(25-30 hours preferred).

After annealing, the steel is generally temper rolled.

A hot roll finishing temperature of at least 1,645F. is necessary toobtain the desired texture at the conclusion of hot rolling. The desiredtexture is one in which the plane of the strip contains a mixture ofcrystal planes and no one crystal plane predominates to any significantdegree in the plane of the strip. This is a random crystallographicorientation.

A hot roll finishing temperature of at least l, 645F.

also reduces precipitation of compounds containing titanium, carbon andnitrogen, and this is desirable.

The coiling temperature should be no greater than about 1,350F. becausea higher temperature would coarsen precipitatedcompounds of titanium,carbon and nitrogen, and this is undesirable.

A cold reduction before annealing of at least 60 per.- cent is necessaryto obtain adesired Tvalue'of at least 1.9 after annealing. Generally,the 7 value increases with increased cold reduction. A cold reductionbelow 60 percent produces an Fvalue below 1.9, and a cold reductionabove 60 percent generally produces an F value greater than 1.9. Thefvalue generally increases In the annealing step, 1,285F. is a criticalminimum temperature. Below this temperature the steel will notrecrystallize during annealing; and recrystalliiation during annealingis essential to obtain the desired an nealing texture and to get rid ofthe texture obtained during cold rolling.

' More specifically, in order to obtain the desired high Tvalue, it isessential to produce, in the annealed cold rolled strip, acrystallographic orientation in which there is a relatively highincidence, in the plane of the strip, of cube or corner l l l crystalplanes and a relatively low incidenc, in the plane of the strip, of cubeafter cold rolling, before annealing; but it can be produced with arecrystallizing anneal.

The previously described random orientation of crystal planes producedby the hot rolling step provides a mixture of cube or corner 1 l lcrystal planes, cube on EXAMPLES Molten steel made by conventionalprocedures, without vacuum degassing, and having the composition setforth in Table I, was cast into ingots using conventional procedure. Thesteel was aluminum killed.

face (100) crystal planes, and all other possible planes, in the planeof the strip.

Not only must there be a recrystallization anneal, as distinguished frommerely a recovery anneal (as in Goodenow, U.S.-Pat. No. 3,492,173), but,also, recrystallization must be' complete in order to obtain the desiredstrip properties. These properties include not only the desired high 7value, but also particular strength and ductility levels.

More specifically, .a given forming operation including drawingtypically also includes stretching. The drawability of the strip isrelated to the 7 value (the higher the 7 value, the better thedrawability). The stretchability of the strip is related to theductility (elongation). Generally, the higher the ductility the betterthe stretchability. Moreover, yield strength (or 0.2 percent proofstress afater temper rolling) is also a factor. Too low a yield strengthis undesirable in many applications because then the strip is too softor too weak structurally. Too high a yield strength may be above thelimitations of the forming press. A typical desired ductility, expressedas total elongation in a 2 inch gage length, would be greater than 35percent, for example. A typical desired range for strength, expressed as0.2 percent proof stress, after a temper roll of Vi- /zpercent, isl5,000-25,000 p.s.i., for example.

The length of time of the anneal is controlled by the length of time ittakes to completely recrystallize the steel and obtain the desiredcrystallographic orientation and the properties described aboveincluding the desired Tvalue. Thus, with a minimum annealing temperatureof 1,285F., there should be at least hours of annealing time at thistemperature to obtain complete recrystallization. The higher theannealing temperature, the shorter the time required, but a limitationon the annealing temperature for a batch anneal is that, when thetemperature is too high, the laps of the coil stick together. Thisdictates an upper limit on the annealing temperature, when the coil isbatch annealed, of about 1,350F.

When annealing utilizing the open coil method, the laps of the coil donttouch and the steel may be heated to a higher annealing temperature thanin the batch anneal method. However, the annealing temperature must bebelow the temperature at which austenite begins to form in theparticular steel composition, and this is so whether annealing with thebatch method or the open coil method.

Continuous annealing methods may also be used, the primary considerationbeing that recrystallization be complete and formation of austeniteavoided.

The. ingots were broken down into slabs using conventional procedure.The slabs were reheated for approximately 2 /z hours at 2,300F., hotrolled and coiled into six coils. The hot rolling finishing temperature,the coiling temperature and the thickness of the coils of hot rolledstrip is reflected in Table II.

TABLE II HOT ROLLING FINISHING AND COILING TEMPERATURES After hotrolling, the strip was cold rolled. The percent of reduction during coldrolling is reflected in Table IV.

After cold rolling, the cold rolled strip was open coiled annealed atabout 1,600F. for a time in the range 26-30 hours. Details of theannealing treatment are given in Table 111.

Some physical properties of the strip after open coil annealing aregiven in Table IV.

Table IV shows that 4 of the 5 samples of cold rolled strip subjected toa cold reduction of about 60 percent or greater (59.5 65.0 percent)before annealing had an r value after annealing greater than 1.9 (andthe fifth sample had an Tvalue of 1.86) and a grain size at least asfine as 8.0 whereas the highest Fvalue for the strip samples subjectedto cold reduction in the range 46.5 57.5 percent was 1.82 (and this wasat 57.5 percent cold reduction).

After annealing, the strips was subjected to further cold reduction ortemper rolling, as shown in Table V which also gives various physicalproperties of the strip. For those samples undergoing a total coldreduction of about 65-70 percent, the mean 7 value was about 1.9.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom as modifications will be obvious to those skilled in the art.

TABLE 111 DETAILS OF OPEN COIL ANNEALING TREATMENT Coil No. Heating RateSoak Temp Soak Time Cooling Rate 60F./hr. I600F. i F. 26 /6 hrs.

3 60F./hr. 1590F. 5F. hrs. 60F./hr. down to 4 1450F. then 200F./hr. downto 830F. Remainder of cycle at F./hr.

. or less 5 60F./hr. I600F. t 5F. 29 hrs.

" Taken from hearth thermocouple.

' A jump above 1615F. for approximately 1 hour occurred during the soak.Maximum temperature reached was 1640F.

TABLE 1v MECHANICAL PROPERTIES AND GRAIN SIZE AFTER OPEN COIL ANNEALINGCold Yield Tensile Elongation Elongation Hard- Coil (1) Test ThicknessReduction Grain Size, Stren th Stren h Uniform Total ness. No. PositionInches ASTM No. X 15 X l 3 (2), (3) (2), (3) T RB psi (3) psi (3) 1 Head.075 53.0 7.5 8.5 15.8 46.2 26.2 45.6 1.73 40.6 Tail .085 46.5 7.5 8.515.9 45.4 28.7 46.5 1.65 43.5

3 Hd .061 62.0 8.5 9.5 17.4 48.0 25.1 45.3 1.96 44.3 Tl .068 57.5 8.08.5 17.5 48.1 24.3 44.8 1.82 47.4

4 Hd .065 59.5 8.0 8.5 16.6 47.7 25.3 44.0 1.91 40.2 T1 .073 54.5 8.08.5 17.4 48.2 24.4 45.9 1.79 44.2

5 PM .046 54.0 8.5 9.0 16.2 47.4 25.2 45.4 1.80 39.0 Tl .0355 64.5 8.59.0 16.8 47.7 25.6 43.1 1.95 43.0

(l) Coils 1 through 4 tandem rolled fromm 0.170 inch Hot Rolled Strip.Coils 5 and 6 tandem rolled from 0.100 inch Hot Rolled Strip (2)Measured in a 2" gauge length. (3) Average of 6 samples (2 longitudinal,2 diagonal and 2 transverse).

TABLE V MECHANICAL PROPERTIES AFTER TEMPER ROLLING 0.2% Proof TensileElongation, Elongation, Stress X 10 Strength B Uniform otal Total ColdCoil Test Thickness, psi. X 10 psi. (1), (2) (1), (2) T Reduction No.Position Inches (2) (2) '7? 1 Head 056 24.1 48.0 24.8 43.7 1.89 67.0Tail .059 29.3 47.6 25.0 44.2 1.90 65.2 2 Hd .058 25.3 48.8 25.0 44.31.92 65.8 T] .059 24.1 48.8 24.7 45.4 1.98 65.8 3 Hd .060 26.2 49.2 24.445.0 1.98 64.8 T1 .057- 26.4 48.5 23.9 44.5 2.03 66.5 4 Hd .057 24.547.8 25.0 44.0 1.89 66.5 T1 .058 23.7 47.6 25.0 44.0 1.93 65.8 5 Hd .040280 47.2 24.6 43.6 1.82 60.0

T1 .035 25.8 47.4 24.7 42.8 1.89 65.0 6 Hd .035 27.2 46.2 24.8 42.3 1.8365.0 T1 .035 25.1 46.3 24.7 43.3 1.83 65.0

(1) Measured in a 2" gauge length. (2) Average of 6 samples (2longitudinal. 2 diagonal and 2 transverse).

What is claimed is:

1. A method for producing a finished steel strip with improved deepdrawing properties, said method comprising the steps of:

providing a solidified killed steel consisting essentially of 0.04 -0.06wt. percent carbon, 0.02-0.50 wt. percent manganese, less than 0.015 wt.percent nitrogen, titanium in an amount by weight at least six times thecombined carbon and nitrogen content, and a balance consistingessentially of iron; hot rolling said steel into hot rolled strip, witha hot roll finishing temperature greater than 1,645F.; cold rolling saidhot rolled strip into cold rolled strip with acold reduction of at least60 percent; annealing said cold rolled strip, at a temperature exceeding1,285F. and below the temperature at which austenite begins to form insaid steel, without decarburizing, for a time sufficient to completelyrecrystallize the cold rolled strip and produce a crystallographicorientation in which there is a relatively high incidence, :in the planeof the strip, of cube on corner crystal planes and a relatively lowincidence, in the plane of the strip, of cube on face crystal planes;

said strip having a grain size, after annealing, at least as fine as 8.0and a carbon content, after annealing, essentially the same as thecarbon content before annealing;

and temper rolling after annealing;

said finished steel strip having an average plastic strain ratio (7) ofat least 1.9 and a strength, expressed as 0.2 percent proof stress, inthe range l5,00025,000 psi. v

2. A method as recited in claim I and comprising:

coiling said hot rolled strip at the conclusion of said hot rollingstep, with a strip temperature, at coiling, of less than l,350F.

3. A method as recited in claim 1 wherein said steel is aluminum killedand consists essentially of, in wt. percent:

0.04 0.06 carbon 0.29 0.35 manganese 0.003 0.007 nitrogen 0.30 0.65titanium 0.026 0.040 aluminum and a balance consisting essentially ofiron.

4. A method as recited in claim 1 wherein said steel is hot rolled toproduce hot rolled strip having a random crystallographic orientation.

5. A method as recited in claim 1 wherein said hot roll finishingtemperature is in the range 1,645F. 1,710F.

6. A method as recited in claim 1 wherein said annealing step is acontinuous anneal.

7. A method as recited in claim 1 wherein said temper rolling is in therange fir-V2 percent.

8. A method as recited in claim 1 wherein said steel is produced withoutvacuum degassing.

9. A method as recited in claim 1 wherein the total cold reduction towhich said cold rolled strip is subjected is at least 65 percent.

10. A method as recited in claim 1 wherein said cold rolled strip iscoiled and then box annealed for at least 20 hours.

11. A method as recited in claim wherein said cold rolled strip is boxannealed at a temperature in the range l,320F. 1,350F. for a time in therange 25-30 hours.

12. A method as recited in claim 1 wherein said cold rolled strip isopen coil annealed, with an inert atmosphere, at a temperature in therange 1,500F 1,640F. for at least 5 hours.

13. A method as recited in claim 12 wherein said cold rolled strip isopen coil annealed at a temperature in the range 1,590F. 1,620F. for atime up to 30 hours.

15. Cold rolled steel strip as recited in claim 14 and having a steelcomposition consisting essentially of, in wt. percent: 1

0.04 0.06 carbon,

0.29 0.35 manganese,

0.003 0.007 nitrogen,

0.03 0.65 titanium,

0.026 0.040 aluminum,

and a balance consisting essentially of iron.

16. Cold rolled steel strip as recited in claim 14 and having thefollowing physical properties:

ductility, expressed as total elongation in a two inch gage length, ofat least 35 percent.

1. A METHOD FFOR PRODUCING A FINISHED STEEL STRIP WITH IMPROVED DEEPDRAWING PROPERTIES, SAID METHOD COMPRISING THE STEPS OF: PROVIDING ASOLIDIFIED KILLED STEEL CONSISTING ESSENTIALLY OF 0.04 -0.006 WT.PERCENT CARBON , 0.02-0.50 WT. PERCENY MANGANESE, LESS THAN 0.015 WT.PERCENT NITROGEN, TITANIUM IN AN AMOUNT BY WEIGHT AT LEAST SIX TIMES THECOMBINED CARBON AND NITROGEN CONTENT, AND A BALANCE CONSISTINGESSENTIALLY OF IRON; HOT ROLLING SAID STEEL INTO HOT ROLLED STRIP, WITHA HOT ROLL FINISHING TEMPERATURE GREATER THAN 1,645*F; COLD ROLLING SAIDHOT ROLLED STRIP INTO COLD ROLLED STRIP WITH A COLD REDUCTION OF ATLEAST 60 PERCENT; ANNEALING SAID COLD ROLLED STRIP, AT A TEMPERATUREEXCEEDING 1,285*F. AND BELOW THE TEMPERATURE AT WHICH AUSTENITE BEGINSTO FORM IN SAID STEEL, WITHOUT DECARBURIZING, FOR A TIME SUFFICIENT TOCOMPLETELY RECRYSTALLIZE THE COLD ROLLED STRIP AND PRODUCE ACRYSTALLOGRAPHIC ORIENTATION IN WHICH THERE IS A RELATIVELY HIGHINCIDENCE, IN THE PLANE OF THE STRIP, OF CUBE ON CORNER CRYSTAL PLANESAND A RELATIVELY LOW INCIDENCE, IN THE PLANE OF THE STRIP, OF CUBE OFFACE CRYSTAL PLANES; SAID STRIP HAVING A GRAIN SIZE ANNEALING, AT LASTAS FINE AS 8.0 AND A CARBON CONTENT, AFTER ANNEALING ESSENTIALLY THESAME AS THE CARBON CONTENT BEFORE ANNEALING; AND TEMPER ROLLING AFTERANNEALING; SAID FINISHED STEEL STRIP HAVING AN AVERAGE PLASTIC STRAINRATI O (R) OF AT LEAST 1.9 AND A STRENGTH, EXPRESSED AS 0.2 PERCENTPROOF STRESS, IN THE RANGE 15,000-25,000 PSI.
 2. A method as recited inclaim 1 and comprising: coiling said hot rolled strip at the conclusionof said hot rolling step, with a strip temperature, at coiling, of lessthan 1,350*F.
 3. A method as recited in claim 1 wherein said steel isaluminum killed and consists essentially of, in wt. percent:
 4. A methodas recited in claim 1 wherein said steel is hot rolled to produce hotrolled strip having a random crystallographic orientation.
 5. A methodas recited in claim 1 wherein said hot roll finishing temperature is inthe range 1,645*F. - 1,710*F.
 6. A method as recited in claim 1 whereinsaid annealing step is a continuous anneal.
 7. A method as recited inclaim 1 wherein said temper rolling is in the range 1/4 - 1/2 percent.8. A method as recited in claim 1 wherein said steel is produced withoutvacuum degassing.
 9. A method as recited in claim 1 wherein the totalcold reduction to which said cold rolled strip is subjected is at least65 percent.
 10. A method as recited in claim 1 wherein said cold rolledstrip is coiled and then box annealed for at least 20 hours.
 11. Amethod as recited in claim 10 wherein said cold rolled strip is boxannealed at a temperature in the range 1,320*F. - 1, 350*F. for a timein the range 25-30 hours.
 12. A method as recited in claim 1 whereinsaid cold rolled strip is open coil annealed, with an inert atmosphere,at a temperature in the range 1,500*F. - 1,640*F. for at least 5 hours.13. A method as recited in claim 12 wherein said cold rolled strip isopen coil annealed at a temperature in the range 1, 590*F. - 1,620*F.for a time up to 30 hours.
 14. Cold rolled steel strip having deepdrawing properties, said strip being composed of killed steel consistingessentially of, in wt. percent: 0.04 - 0.06 carbon, 0.02 - 0.50manganese, less than 0.015 nitrogen, titanium in an amount at least sixtimes the combined carbon and nitrogen content, and a balance consistingessentially of iron; said cold rolled steel strip having an averageplastic strain ratio (r) of about 1.9 or more, strength, expressed as0.2 proof stress, in the range 15,000-25,000 psi, and a grain size of atleast 8.0.
 15. Cold rolled steel strip as recited in claim 14 and havinga steel composition consisting essentially of, in wt. percent: 0.04 -0.06 carbon, 0.29 - 0.35 manganese, 0.003 - 0.007 nitrogen, 0.03 - 0.65titanium, 0.026 - 0.040 aluminum, and a balance consisting essentiallyof iron.
 16. Cold rolled steel strip as recited in claim 14 and havingthe following physical properties: ductility, expressed as totalelongation in a two inch gage length, of at least 35 percent.